Support for generating pep484 .pyi stubs

[Static typing in python](https://realpython.com/python-type-checking/) (aka PEP-484) is gaining a lot of traction, with linting tools like [mypy](https://mypy.readthedocs.io/en/latest/) reaching maturity, and support in IDEs -- like PyCharm, VSCode, and Sublime -- for using pyi stubs to drive code inspection and completion. Stubs are particularly impactful for c/c++ bindings which are opaque to static inspectors.

pyi stub generation is already supported by other python binding generators like pybind11, shiboken, and sip.

Has this feature been discussed at all in boost-dev circles?